This invention relates generally to systems for producing responses to ambient light which corresponds to the response of a human eye, and more particularly, to a system which is particularly adaptable to the control of headlamps in vehicles in response to changes in the incident ambient light.
Headlamp control systems for use in vehicles, which systems are also known as "twilight sensors," have several shortcomings, most of which are derived from the fact that a known twilight sensor responds differently to ambient light than a human eye. As a result of this different response characteristic, twilight sensors will respond differently than the eye as each is subjected to different lighting conditions.
As an example of the foregoing, assume, for the moment, that a twilight sensor is adjusted to actuate the headlamps of a vehicle at 200 foot-candles under a cloudless sky. Such a setting might cause the headlamps to be turned on at 100 foot-candles on a cloudy day. A foot-candle is a unit of light intensity which takes into account the spectral response of the human eye to light. As is known, the human eye is most sensitive to green light, and therefore less light energy in the form of green light is required to obtain the same response to the eye, as, for example, red light.
One known twilight sensor system employs a blue filter followed by a photoresistive cell, or a photodiode, whereby the light sensor will respond only to blue light. The blue filter is intended to reduce the sensitivity of the twilight sensor to street lights which radiate mostly yellow and red light. Clearly, this known twilight sensor system will not correspond in its operation to the light which is seen by a human eye.
It is, therefore, an object of this invention to provide a system which is responsive to ambient light in accordance with a characteristic which corresponds to that of a human eye.
It is another object of this invention to provide a system for controlling the operation of the headlamps of a vehicle, whereby the headlamps are operated in a manner consistent with the perception of the ambient light by a human operator of the vehicle.
In addition to the foregoing, there is a need for a twilight sensor to respond quickly to the rapid changes in the intensity of ambient light, particularly when the ambient light rapidly becomes diminished, as would be the case when a vehicle which enters a tunnel. Thus, it is also an object of this invention to provide a twilight sensor arrangement which can respond quickly to actuate the headlamps when the ambient light diminishes rapidly, without responding to electrical noise or transient ambient light.
It is a further object of this invention to provide a headlamp control system for a vehicle wherein it will always appear to the driver that the headlamps are turning on when the sky reaches the same light brightness.
It is additionally an object of this invention to provide a headlamp control system which actuates the headlamps of a vehicle in a manner consistent with the perception of a human eye, irrespective of whether the ambient conditions are clear or cloudy.